Prior-art optical burst switching has two main drawbacks: burst-transfer latency and burst loss. In a closed-loop scheme, a source node sends a request to a core node for transferring a burst, the request including a destination and size of the burst, and waits for a message from the core node, where the message acknowledges that the optical switch in the core node is properly configured, before sending the burst. In an open-loop scheme, the burst follows the burst transfer request after a predetermined time period, presumably sufficient to schedule the burst transfer across the core, and it is expected that, when the burst arrives at the core node, the controller of the core node would have set up an internal path through the optical switch to a target output port of the optical switch. The main drawback of this technique is the uncertainty of the fate of a burst thus transmitted. Even if a very long time gap is kept between a burst-transfer request and the data burst itself, the lack of buffers at the core node may result in burst loss and a significant idle time.
Thus, in the closed-loop scheme, the time delay experienced in sending a burst transfer request and receiving an acceptance before sending a burst may be unacceptably high, leading to idle waiting periods and low network utilization in addition to requiring large storage at the edge nodes. In the open-loop scheme, a burst may arrive at a core node before the optical switch can be configured to switch the burst and the burst may be lost. Furthermore, the fact that the burst has been lost at the core node remains unknown to the source node for some time and a lost burst would have to be sent again after a predefined interval of time.
In a wide-coverage network, the round-trip propagation delay from an edge node, comprising a paired source node and a sink node, to a core node can be of the order of tens of milliseconds. This renders closed-loop burst scheduling inappropriate. In closed-loop switching, a source node and a core node must exchange messages to determine the transmission time of each burst. The high round-trip delay requires that the source node have sizeable buffer storage. On the other hand, open-loop burst scheduling, which overcomes the delay problem, can result in substantial burst loss due to unresolved contention at the core nodes. It is desirable that data bursts formation at the source nodes and subsequent transfer to respective optical core nodes be performed with low delay, and that burst transfer across the core be strictly loss-free. It is also desirable that the processing effort and transport overhead be negligibly small.
Applicant's U.S. patent application Ser. No. 09/750,071, filed on Dec. 29, 2000 and titled “Burst Switching in a High-Capacity Network”, discloses a method of burst switching where burst transfer requests from edge nodes are sent to a core-node controller which determines a schedule for conflict-free burst switching through the core node. Scheduling information is distributed to the sources of the burst transfer requests and to a configuration controller of the core node. Instead of handling burst requests one-by-one, burst requests are pipelined and the handling of the bursts is scheduled over a future period, thus realizing efficient utilization of network resources.
Applicant's copending U.S. patent application Ser. No. 10/054,509, filed on Nov. 13, 2001 and titled “Time-Coordination in a Burst-Switching Network”, discloses a method and apparatus for low latency loss-free burst switching. Burst schedules are initiated by controllers of bufferless core nodes and distributed to respective edge nodes. Burst formation takes place at source nodes and a burst size is determined according to an allocated flow rate of a burst stream to which the burst belongs. An allocated flow rate of a burst stream may be modified according to observed usage of scheduled bursts of a burst stream. A method of control-burst exchange between each of a plurality of edge nodes and each of a plurality of bufferless core nodes enables burst scheduling, time coordination, and loss-free burst switching.
The method of application Ser. No. 09/750,071 generally yields higher network efficiency and is attractive when the propagation delay between an edge node and a core node is relatively small, of the order of a millisecond for example, or when large delay is acceptable. The method of application Ser. No. 10/054,509 is attractive when the propagation delay between an edge node and a core node is relatively large.
Burst communication is preferably incorporated within a flexible time-shared network that also provides both channel switching and time-division-multiplexed switching. Providing high-capacity wide-coverage time-shared networks using bufferless core nodes presents a significant challenge due to the difficulty of time-alignment at the bufferless core nodes. Structures of time-shared networks that facilitate extending network coverage and capacity are needed.
Furthermore, methods for burst switching adapted to take into account widely varying inter-nodal propagation delays, different network topologies, and diverse performance requirements are needed to realize a wide-coverage high-performance time-shared network.
It is also desirable to explore means for dynamic division of a parent network into embedded networks serving user communities having different service specifications or requiring private control.